A novel active damping scheme for use with regenerative converters

Regenerative drives, also known as four-quadrant drives are integral part of many industrial applications ranging from wind power generation, elevators, escalators, pump jacks used in oil fields to centrifuges, etc. All regenerative converters need passive LCL filters to filter out PWM frequency components. One disadvantage of using an input LCL filter in front of regenerative converters is that they draw capacitive current even when the converter is not in operation. The circulating current causes power loss in the power delivery equipment and can significantly affect the overall system efficiency when there are large number of large power regenerative units in the power system. The filter capacitors also interact with pre-existing voltage distortion in the power system. Multiple regenerative units of various sizes have been shown in this paper to cause undesirable voltage ripple which in turn causes current oscillations especially in smaller sized regenerative converters. A new active damping scheme is proposed which also includes actively opening the neutral point of the filter capacitor to eliminate standby power loss associated with regenerative converters with LCL filters. Two ideas are proposed. In the first, only one damping resistor is used while in the other two damping resistors are used in parallel. One of the resistor acts as the fixed damping resistor while the other is modulated depending on the amount of system distortion. Test results are given to demonstrate the fundamental concept.